Cookies and Concurrency, Redux

In yesterday’s episode, I shared the root cause of a bug that can cause document.cookie to incorrectly return an empty string if the cookie is over 1kb and the cookie grows in the middle of a DOM document.cookie getter operation.

Unfortunately, that simple bug wasn’t the root cause of the compatibility problem that I was investigating when my code-review uncovered it. The observed compatibility bug was slightly different– in the repro case, only one of the document’s cookies goes missing, and it goes missing even when only one page is setting the cookie.

After the brain-melting exercise of annotating the site’s minified framework libraries (console.log(‘…’) ftw!) via Fiddler’s AutoResponder, I found that the site uses the document.cookie API to save the same cookie (named “ld“) three times in a row, adding some information to the cookie each time. However, the ld cookie mysteriously disappears between 0.4 and 6 milliseconds after it gets set the third time. I painstakingly verified that the cookie wasn’t getting manipulated from any other context when it disappeared.


As I wrote up the investigation notes, I idly noted that due to a trivial typo in the website’s source code, the ld cookie was set first as a Persistent cookie, then (accidentally) as a Session cookie, then as a Persistent cookie.

In re-reading the notes an hour later, again my memory got tickled. Hadn’t I seen something like this before?

Indeed, I had. Just about five years ago, a user reported a similar bug where a HTTP response contained two Set-Cookie calls for the same cookie name and Internet Explorer didn’t store either cookie. I built a reduced test case and reported it to the engineering team.

Pushing Cookies

The root cause of the cookie disappearance relates to the Internet Explorer and Edge “loosely-coupled architecture.”

In IE and Edge, each browser tab process runs its own networking stack, in-process1. For persistent cookies, this poses no problem, because every browser process hits the same WinINET cookie storage area and gets back the latest value of the persistent cookie. In contrast, for session cookies, there’s a challenge. Session cookies are stored in local (per-process) variables in the networking code, but a browser session may include multiple tab processes. A Session cookie set in a tab process needs to be available in all other tab processes in that browser session.

As a consequence, when a tab writes a Session cookie, Edge must send an interprocess communication (IPC) message to every other process in the browser session, telling each to update its internal variables with the new value of the Session cookie. This Cookie Pushing IPC is asynchronous, and if the named cookie were later modified in a process before the IPC announcing the earlier update to the cookie is received, that later update is obliterated.

The Duplicate Set-Cookie header version of this bug got fixed in the Fall 2017 Update (RS3) to Windows 10 and thus my old Set-Cookie test case case no longer reproduces the problem.

Unfortunately, it turns out that the RS3 fix only corrected the behavior of the network stack when it encounters this pattern– if the cookie-setting calls are made via document.cookie, the problem reappears, as in this document.cookie test case.


Playing with the repro page, you’ll notice that manually pushing “Set HOT as a Session cookie” or “Set as a Persistent cookie” works fine, because your puny human reflexes aren’t faster than the cookie-pushing IPC. But when you push the “Set twice” button that sets the cookie twice in fast succession, the HOT cookie disappears in Edge (and in IE11, if you have more than one tab open).

Until this bug is fixed, avoid using document.cookie to change a persistent cookie to a session cookie.


In contrast, in Chrome, all networking occurs in the browser process (or a networking-only process), and if a tab process wants to get the current document.cookie, it must perform an IPC to ask the browser process for the cookie value. We call this “cookie pulling.”

Published by ericlaw

Impatient optimist. Dad. Author/speaker. Created Fiddler & SlickRun. PM @ Microsoft 2001-2012, and 2018-2022, working on Office, IE, and Edge. Now a SWE on Microsoft Defender Web Protection. My words are my own, I do not speak for any other entity.

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